The present invention pertains to security systems such as personnel monitoring systems and also pertains to components and methods useful in connection with such systems.
Automated systems have been developed for monitoring persons placed under house arrest. Such automated systems are intended to verify the presence of the monitored individual at his home or at another specified location. With such a system, a person can be sentenced to house arrest either continuously or during specified hours of the day as a punishment for crime. Such a sentence may be employed either as an alternative to incarceration in a conventional jail or as an alternative to an ordinary parole program. House arrest is widely regarded as a desirable alternative to conventional incarceration for offenders other than the most hardened, dangerous criminals. House arrest avoids the costs associated with conventional jails and also avoids the adverse effects on the inmate of exposure to other criminals while in jail. Additionally, a person sentenced to house arrest can maintain his contacts with his family and neighborhood and may also maintain his employment, thus facilitating his re-integration into society. House arrest with automated monitoring is also regarded as a useful alternative to conventional parole programs. An automatic monitoring system can provide substantially continuous monitoring of the offender's behavior which cannot be provided by any practical program of personal visits by parole officers.
Many of the automated monitoring systems employed heretofore use a computer installed at a central station in conjunction with devices at monitoring locations remote from the central station, such as in the individual parolee's home. Each parolee may wear a tag permanently secured to his body such as a wristlet or anklet. Each tag bears unique encoded information. Each remote device is arranged to respond to the tag of the appropriate parolee and to verify to the central computer that the tag, and hence the parolee is present at the remote location. A system described in copending, commonly assigned U.S. Pat. application No. 765,343 filed Aug. 13, 1985, now U.S. Pat. No. 4,747,120 employs passive encoded tags attached to each parolee. The central computer periodically places telephone calls to the home of each parolee and signals the parolee to insert the tag into a tag verifying device at his home. If the correct tag is inserted into the tag verifying device in response to the signal sent by the central computer, the device sends an appropriate return signal to the central computer. If the central computer does not receive such a return signal in response to its call, the central computer can generate an appropriate alarm signal indicating that the parolee is not present. Other parolee monitoring systems employ a small, low-powered radio frequency or "RF" transmitter in each tag. Each such transmitter is arranged to send an RF signal bearing a unique identifying code. The remote monitoring device at each parolee's home incorporates an RF receiver and means for detecting receipt of the appropriate encoded RF signal. If the remote monitoring device does not receive the appropriate monitoring signal, it automatically places a telephone call to the central computer and sends a message to the central computer indicating that the parolee is absent.
These systems require that the tag remain attached to the parolee so that presence of the tag at the remote location necessarily will indicate that the parolee is present. Systems employing passive tags typically utilize a "tamper evident" securement strap, such that the tag cannot be removed without permanently deforming or damaging some physical element of the tag. Periodic inspections of the tag by a parole officer suffice to insure that the tag has been secured to the parolee at all times between such inspections. Systems using RF transmitting tags typically provide a sensor in the tag itself to alter operation of the transmitter if the tag is removed from the body of the parolee, so that the remote monitoring device will call the central computer and report a violation if the parolee removes the tag from his body. Ordinarily, the tag is attached to the parolee's body by a strap connected to the transmitter housing and having its ends connected to form a closed loop encircling the parolee's wrist or ankle. The strap includes a conductive element, and an electronic sensing circuit continually passes a current through the conductive element of the strap. This circuit is arranged to detect any cessation of the current flow and thus detect any breakage or disconnection of the strap. In this event, the circuit switches the transmitter to a tamper mode, wherein the transmitter does not send the normal signal. An electronic latch circuit is provided to keep the transmitter in tamper mode after even a momentary interruption of the current flow through the conductor in the strap, so that the system cannot be defeated by cutting the strap and then reconnecting it. Systems of this type supposedly provide better security than provided by a tamper evident securement with periodic visual inspection.
Unfortunately, systems of this type have been unreliable and susceptible to cheating. The sensing circuit must be powered by a self-contained power source such as a small storage battery. To provide acceptable battery life, the sensing circuit must operate with a very small current flow through the conductor in the strap. Such low current, high impedance circuits typically are susceptible to interference from stray electromagnetic fields and hence may provide a false indication of tampering. Moreover, the sensing circuit typically places the transmitter in tamper mode while the tag is being attached to the parolee by the parole officer. The loop must be open so that the parole officer can place the tag on the parolee's wrist or ankle. Therefore, a reset device must be built into the transmitter. After securing the tag on the parolee, the parole officer activates the reset device, thus overriding the latch circuit and restoring the transmitter to normal mode. Such reset devices severely compromise the security of the system. The parolee necessarily must be present during resetting and hence can observe the resetting procedure. The parolee often can make an educated guess as to the mechanism used to reset the transmitter. He can then actuate the reset device himself and defeat the system.
Moreover, RF systems utilized heretofore can be defeated by counterfeiting the RF signals. RF tag systems utilized heretofore ordinarily employ standardized radio transmitters such as those employed with radio actuated garage door openers, home security systems and the like. Although it would be theoretically possible to develop entirely unique transmitters and receivers for personnel monitoring systems so as to provide a counterfeit-resistant signal with many unique characteristics, the costs of such specialized development and the costs of the required government approval procedures for entirely new transmitter designs render this approach impractical. The security problems posed by use of a standardized transmitter design are aggravated by government labeling requirements. In the United States, any radio transmitter must be labeled with certain required information. This information can be used by an educated criminal to find other commercially available transmitters useful in a counterfeiting effort. The transmitter in the tag typically is arranged to apply a supposedly unique digital code to the transmitted signal. However, there are only a limited number of such codes, and the code used by a particular transmitter can be determined by experimental work. Once the parolee has an appropriately set spurious transmitter, he may violate his parole without detection by the system.
The remote monitoring devices employed in parolee monitoring systems typically share the telephone lines at the parolee's home with the standard telephone instruments. The remote monitoring device typically is provided with features for discriminating between various conditions on the telephone line, including a "off-hook" condition wherein a standard handset connected to the line is in use, and a "on-hook" condition wherein no telephone is in use on the line. It may also be necessary to detect a disconnected condition wherein the line is disconnected from the telephone network. Most telephone systems are arranged so that there is a substantial voltage present when the handset is on-hook, and a lesser voltage present when the handset is off-hook, whereas no voltage is present when the line is disconnected. Accordingly, the discrimination apparatus utilized heretofore has employed voltage sensitive test means operating according to a "voltage window" principle. Thus, a voltage on the line above a first, relatively high threshold indicates an on-hook condition, whereas a voltage below this threshold but above another, lower threshold indicates an off-hook condition and a voltage below the lowest threshold indicates a line disconnected condition. The line condition discrimination apparatus utilized heretofore typically has had very high input impedance so as to limit the current drawn from the telephone line as required by government and telephone company regulations. These systems have been unreliable.
Thus, there have been substantial, unmet needs heretofore for improvement in personnel monitoring systems and in components useful in these and related systems.